1. Field of the Invention
The present invention relates to a method of making thin film transistors and, more particularly, to a method of making thin film transistors using silicon thin films.
2. Description of the Prior Art
In a static random access memory (SRAM) of high resistance load type or the like, in order to assure an operation margin, reliability, a stand-by current and so forth, there has been proposed a stack type SRAM that uses a thin film transistor (TFT) formed on a polycrystalline silicon which is superior in uniformity of the film quality as a load element.
Polycrystalline silicon can be formed by an ordinary chemical vapor deposition (CVD) method. Particularly, if it is intended to form a polycrystalline silicon film having crystal grains of a relatively large size, then it is difficult to form a polycrystalline silicon film which is superior in uniformity of the film quality and has a high carrier mobility with a low leak. Meanwhile, there is proposed a random solid phase growing method or a selective growing method in which a selective implantation of ions is performed at a high dose by using a resist mask after the implantation of ions was performed at a low dose to thereby produce a core from which crystal is to be grown and then a low temperature solid phase growing processing is carried out. According to such solid phase crystallization (SPC) technique, a grain size of polycrystalline silicon can be increased so that a high carrier mobility is made possible. Therefore, the application, research and trial manufacturing of the above-mentioned TFT stack-type SRAM or the like are expanded by this solid phase crystallization technique.
Meanwhile, with the random solid phase growing method, since it is difficult to grow crystal selectively, it sometimes occurs that a channel of a transistor extends to a grain boundary. If this actually occurs, then a dispersion occurs in leak current or threshold voltage, which deteriorates the reliability of the transistor. Moreover, with the method of selectively forming a single crystal silicon region, also the problems of pollution of resist are involved and a low dose region is non-uniform, which deteriorates the uniformity of the film quality.
The assignee of the present application has previously proposed a semiconductor crystal growing method (see Japanese patent application No. 3-285702). With this previously-proposed method, after a shield mask was formed on the upper face of an amorphous semiconductor layer formed on a substrate, an excimer laser light is irradiated on the amorphous semiconductor layer through the shield mask to produce on the amorphous semiconductor layer a core from which crystal is to be grown, whereafter crystal is grown from the core for the crystallization by a low temperature solid phase annealing process to thereby form a single crystal region on the amorphous semiconductor layer.
According to the aforesaid method, the single crystal region of high quality can be selectively grown on the amorphous semiconductor layer and the channel layer of the transistor cannot be extended to the grain boundary. Accordingly, a leak current can be reduced considerably and a carrier mobility can be increased. Also, the fluctuation of a threshold voltage can be suppressed and the transistor thus produced is high in reliability.